it's not obviously nonsense to posit a mental substance that simply doesn't reduce to anything else, in the way that it's obviously nonsense to posit that the law of noncontradiction is false.

Perhaps I haven't been clear enough. Oh, I agree with your statement. It's just that, even if all of the obstacles to such a hypothesis were overcome, it wouldn't actually explain anything. It would just let us know where we have to focus our investigations. People who insist that humans are somehow fundamentally different from the rest of the universe don't usually grasp that.